Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulposus, which is surrounded and confined by an outer, fibrous ring called the annulus fibrosis. In a healthy, undamaged spine, the annulus fibrosis prevents the nucleus pulposus from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease or aging. Disruption of the annulus fibrosis allows the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on the spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
Sometimes the only relief from the symptoms of these conditions is a discectomy, or surgical removal of a portion or all of an intervertebral disc followed by fusion of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain.
Bone grafts are often used to fill the intervertebral space to prevent disc space collapse and promote fusion of the adjacent vertebrae across the disc space. For example, in the Smith-Robinson technique for cervical fusion, the surgeon prepares the endplates of the adjacent vertebral bodies to accept a graft after the disc has been removed. The endplates are generally prepared to be parallel surfaces with a high speed burr. The surgeon sculpts the graft to fit tightly between the bone surfaces so that the graft is held by compression between the vertebral bodies. The bone graft is intended to provide structural support and promote bone ingrowth to achieve a solid fusion of the affected joint.
Unfortunately, the use of bone grafts presents several disadvantages. Autografts, bone material surgically removed from the patient, can be undesirable because they may not yield a sufficient quantity of graft material. The additional surgery to extract the autograft also increases the risk of infection and blood loss. The structural integrity at the donor site can be reduced. Furthermore, some patients complain that the graft harvesting surgery is more painful than the fusion surgery.
Allograft material, which is obtained from donors of the same species, is more readily obtained. However, allografts can be disadvantageous because of disease transmission, immune reactions and religious objections. Furthermore, allogenic bone does not have the osteoinductive potential of autogenous bone and therefore may provide only temporary support.
Both allograft and autograft present additional difficulties. Graft alone may not provide the stability required to withstand spinal loads. Internal fixation can address this problem but presents its own disadvantages such as the need for more complex surgery. Also, the surgeon is often required to repeatedly trim the graft material to obtain the correct size to fill and stabilize the disc space. This trial and error approach increases the length of time required for surgery. Furthermore, the graft material usually has a smooth surface which does not provide a good friction fit between the adjacent vertebrae. Slippage of the graft may cause neural and vascular injury, as well as collapse of the disc space.
Prosthetic implants can be used to prevent collapse of the disc space. The implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the compressive loads on the spine for the life of the patient.
Many attempts to restore the intervertebral disc space after removal of the disc have relied on metal devices. U.S. Pat. No. 4,878,915 to Brantigan teaches a solid metal plug. U.S. Pat. Nos. 5,044,104; 5,026,373 and 4,961,740 to Ray; 5,015,247 to Michelson and U.S. Pat. No. 4,820,305 to Harms et al., U.S. Pat. No. 5,147,402 to Bohler et al. and 5,192,327 to Brantigan teach hollow metal cage structures. Unfortunately, there are several disadvantages associated with the use of these metal implants. For example, metal implants may stress shield the bone graft, increasing the time required for fusion to occur.
Most of the prior implants do not adequately address the need for obtaining a solid fusion. Solid body metal implants do not allow bone ingrowth which may lead to the eventual failure of the implant. Surface porosity in such solid implants may not correct this problem because it often will not allow sufficient ingrowth to provide a solid bone mass strong enough to withstand the loads of the spine. On the other hand, the hollow cage structures of Harms, Ray, Michelson, Bohler and Brantigen allow ingrowth. These devices can also be filled with bone graft material to promote bone growth needed for solid fusion. However, the large openings disclosed in these references, while allowing bone ingrowth, reduce the amount of metal providing structural support, which can limit the implant's load bearing capability until fusion occurs.
Unfortunately, many of these metal devices are also difficult to machine and therefore expensive. For example, the superior and inferior surfaces of Brantigan (U.S. Pat. No. 5,192,327) define ridges for interdigitation with adjacent implants for stacking and biting into the endplates of adjoining vertebrae. Bohler (U.S. Pat. No. 5,147,402) discloses that the surface of the implant can be roughened to promote fusion. These features require more expensive machining and may also compromise the strength of the implant. Moreover the structure of these types of implants do not readily lend themselves for manufacture in smaller sizes for use in the cervical spine.